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A new metamaterial refractive index sensor in terahertz frequency band is proposed. The designed structure is multilayer and periodic. It consists of two SiO 2 layers separated by an air gap. On the upper SiO 2 layer, a unit cell of a graphene circular ring as well as four graphene elliptical half rings are placed while, a graphene circular disk is located on the lower SiO 2 layer. By changing the Fermi level of graphene, selection of the optimized dimensions of the structure and the embedded air gap between the two SiO 2 layers, a high sensitivity sensor by suitable figure of merit is provided. The maximum sensitivity of 36.7 μ m/RIU and figure of merit of 6.43, with refractive index variations of the sensing environment from 1 to 2, is obtained. The symmetric sensor is independence from the incident wave polarization and angle. Electric field and surface current distribution of the structure is investigated and strong …